The invention relates generally to x-ray tubes and, more particularly, to a ferrofluid seal in an x-ray tube and a method of assembling same.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is typically necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which typically includes tool steel ball bearings and tool steel raceways positioned within the vacuum region, thereby requiring lubrication by a solid lubricant such as silver. In addition, the rotor, as well, is placed in the vacuum region of the x-ray tube. Wear of the silver and loss thereof from the bearing contact region increases acoustic noise and slows the rotor during operation. Placement of the bearing assembly in the vacuum region prevents lubricating with wet bearing lubricants, such as grease or oil, and performing maintenance on the bearing assembly to replace the solid lubricant.
In addition, the operating conditions of newer generation x-ray tubes have become increasingly aggressive in terms of stresses because of G forces imposed by higher gantry speeds and higher anode run speeds. As a result, there is greater emphasis in finding bearing solutions for improved performance under the more stringent operating conditions. Placing the bearing assembly and rotor outside the vacuum region of the x-ray tube by use of a hermetic rotating seal such as a ferrofluid seal allows the use of wet lubricants, such as grease or oil, to lubricate the bearing assembly.
A ferrofluid seal typically includes a series of annular regions between a rotating component and a non-rotating component. The annular regions are occupied by a ferrofluid that is typically a hydrocarbon-based or fluorocarbon-based oil with a suspension of magnetic particles therein. The particles are coated with a stabilizing agent, or surfactant, which prevents agglomeration of the particles and allows the particles to remain in suspension in the matrix fluid. When in the presence of a magnetic field, the ferrofluid is polarized and is caused to form a seal between each of the annular regions. The seal on each annular region, or stage, can separately withstand pressure of typically 1-3 psi and, when each stage is placed in series, the overall assembly can withstand pressure varying from atmospheric pressure on one side to high vacuum on the other side.
The ferrofluid seal allows rotation of a shaft therein designed to deliver mechanical power from the motor to the anode. As such, the motor rotor may be placed outside the vacuum region to enable a conventional grease-lubricated or oil-lubricated bearing assembly to be placed on the same side of the seal as the rotor to support the target. Furthermore, such bearings may be larger than those typically used on the vacuum side.
During operation, coolant passing through the shaft may serve as coolant for the conventional bearings or for cooling the ferrofluid seal below its design limit. The target, too, may be cooled via the coolant in the shaft. However, because heat generated in the target passes to the shaft via conduction heat transfer, the amount of heat passing from the target to the shaft may be limited due to thermal resistance at the attachment point between the target and the shaft. The amount of thermal resistance at the attachment point may be affected by the means with which the target is attached to the shaft.
Typically, ferrofluid spindles or assemblies are fabricated and pre-assembled by first attaching bearings to a centershaft, applying the sealing fluid to the centershaft, and then inserting the centershaft, target end first, through an aperture of the assembly from the pressure end of the assembly to the vacuum end of the assembly. However, in order to do so, the target end of centershaft must be smaller than the aperture of the ferrofluid assembly. Thus, the target is typically attached to the centershaft at an attachment point at the end of the shaft after the shaft is first passed through the aperture. Because of proximity of the attachment point to the ferrofluid seal and because the ferrofluid of the seal is limited in the temperature to which it can be raised, attaching the target to the target end of the centershaft precludes attachment via attachment methods that include heating of components—such as welding, brazing, and the like.
Thus, in a typical design, the target is attached to the centershaft via a hole in the target that is no larger than the centershaft. Examples of such attachment may include a threaded end on the centershaft and a matching thread in the target hole at the center of the target or may include a threaded end of the centershaft passing through the hole of the target and having a fastener such as a nut to secure the target to the centershaft. Such joints typically include a thermal resistance at the attachment joint that prevents adequate heat from conducting therethrough, thus serving as a conduction limiter or “bottleneck” in the design.
Therefore, it would be desirable to design an x-ray tube having a ferrofluid assembly therein, and method of assembly thereof, having an improved conduction resistance between the target and the centershaft.